Minimally invasive surgical applicator

ABSTRACT

The present invention provides a minimally invasive surgical applicator device. The device is useful in the application of bone wax during surgical procedures to halt or reduce bone bleeding. The device may be sterilized for re-use or may be made disposable.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. application Ser. No.13/741,786 filed Jan. 15, 2013, which is a continuation of InternationalApplication PCT/US2012/036081 filed on May 2, 2012 which claims thebenefit of U.S. Provisional Patent Application No. 61/481,511 filed onMay 2, 2011, the contents and teachings each of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention provides a minimally invasive applicator devicefor use in surgical settings. The applicator may be used to applytherapeutic or surgical materials, preferably at an elevatedtemperature. Such materials include, for example, sealants, adhesives,therapeutic compositions such as creams, ointments and gels, structuralcomponents such as bone, hydroxyapatite and/or bone wax, particularlymalleable materials and those having temperature dependent phasechanges. The device may be sterilized for re-use or may be madedisposable.

BACKGROUND OF THE INVENTION

Versatility in surgical instruments, especially those used to conductminimally invasive procedures is of greater importance with recentadvancements in the field. Minimally invasive surgical procedures aretypically conducted through small ports and are characterized asendoscopic, laparoscopic, thoracoscopic, and the like. Such proceduresrequire, not only that the instruments be precise but that the sameinstrument performs multiple functions.

Additionally, as surgical techniques become more advanced, surgery isincreasingly being performed through smaller exposures. Minimallyinvasive and robotic surgeries are increasing in numbers across surgicalspecialties, such as neurosurgery, orthopedic surgery, cardiac surgery,thoracic surgery, and otolaryngology. Surgery on many deep portions ofthe body can now be performed through micro-incisions, small tubularports, and robotic arms resulting in better surgical results, fewercomplications, less pain, quicker recovery times, and decreased rates ofinfection. Surgical visualization and precision are even more importantin these surgeries. Consequently, a device which can be adapted toadminister any number of materials or substances to a specific locationof the patient's body both quickly and easily will be preferred over acollection of instruments that perform the same tasks.

It is appreciated that the instruments and their payload be compatiblewith the surgical field. For example, it is often critical that thetemperature of the instruments used be similar to the temperature of theconditional surgical setting, whether very low as in organtransplantation or at normal body temperature or at slightly above roomtemperature. In order to achieve the foregoing, it is often necessary towarm either the instrument or the materials the instruments willdeliver. Ideally the instrument would be designed to warm the materialsbeing used either prior to or during administration or contact with thepatient.

The present invention provides such a surgical instrument. In oneembodiment the present invention finds utility in the application ofbone wax during orthopedic surgery.

A principal requirement of all surgery is hemostasis. When living tissueis incised, bleeding results and without hemostasis, blood loss canresult in significant and life threatening anemia which may requireblood product transfusion and intravenous vasopressor medications toprevent complications such as myocardial infarction, cerebral ischemia,and cardiac arrest. Additionally, bleeding obscures the surgical fieldwhich is of primary concern when surgery is performed though minimalaccess ports and under microsurgical magnification. Impairedvisualization from bleeding can dramatically affect the accuracy,efficiency, safety, and speed of a surgical operation. At the conclusionof the operation, hemostasis is necessary to prevent post-surgicalhematoma formation that can cause neurological deficit (temporary orpermanent), pain, anemia, wound breakdown, and infection; all which canrequire re-operation, cause significant morbidity & mortality, and leadto ballooning health care expenses.

Traditionally, hemostasis is obtained during surgery of soft tissues viaa combination of electrocautery and ligature. However, bone bleedingcannot be electrocauterized nor ligated. Instead, it requiresapplication of bone wax.

Bone is a living and highly vascular tissue. When bone is incised,bleeding can be significant. Bleeding from bone mainly originates fromvenous channels located in the trabecular network. In operationsinvolving the cranium, spine, chest, or other bone structure, bone waxis typically smeared along the bleeding surface to achieve hemostasis.Commonly, the instrument used to apply the bone wax is the surgeon'sindex finger or a blunt surgical dissector. The wax is softened to allowfor it to be rolled into a ball and applied on the tip of a surgicalinstrument where it is then smeared along the bleeding surface. Thisintercalates the wax into the trabecular surface where it provideshemostasis.

The current methods for bone wax application are crude, imprecise, andoften ineffective. For example, during microsurgery of the spine,millimeter scale precision is typically required for surgical maneuversand instruments near the spinal cord and nerve roots. There is no marginof error near the spinal cord as the patient's neurological function canbe permanently compromised by accidental and uncontrolled movementsaround the spinal cord or brain with less force than it takes to strikea key on a keyboard. Using one's finger does not provide the accuracyand control necessary to apply wax in these delicate locations.

Finally, it is known that application of bone wax impairs osteogenesisand can contribute to increased rates of infection. While it isnecessary to obtain hemostasis, improved precision in application shoulddecrease the amount of wax necessary to achieve the same result.

Bone wax is a hemostatic agent which has been used over the past centuryto prevent bleeding. Today, it is mostly comprised of beeswax (72.63%),and is softened with paraffin wax (14.87%) and isopropyl palmitate(12.5%). Bone wax functions by creating a physical barrier that blocksblood flow (ETHICON Bone Wax; MSDS No. Ethicon 151B; Dec. 27, 1989).

Although bone wax is widely accepted as a means to stop bleeding inbone, it has some significant disadvantages. Since bone wax is notbiodegradable, it is not reabsorbed into the body. This unnaturalbarrier inhibits bone regeneration and increases risk of infection(Hoffmann, B., et al. “A New Biodegradable Bone Wax Substitute with thePotential to be used as a Bone Filling Material.” Journal of MaterialsChemistry. 17 (2007): 4028). As excess bone wax can be harmful to apatient's recovery, surgeons attempt to minimize its use. In fact, inprocedures where bone fusion is critical, the use of bone wax is avoidedaltogether (Magyar, C. E., et al. “Ostene, A New Alkylene OxideCopolymer Bone Hemostatic Material, does Not Inhibit Bone Healing.”Neurosurgery 63.4 Suppl 2 (2008): 373.) Alternatives to bone wax, suchas gelatin, microfibrillar collagen, and oxidized regenerated cellulosehave been developed to address these negative properties, but bone waxremains the most widely used hemostatic agent (Schonauer, C., et al.“The use of Local Gents: Bone Wax, Gelatin, Collagen, OxidizedCellulose.” Eur Spine J. 13 (2004): S89).

The term minimally invasive surgery refers to any surgery that uses asmaller than traditional opening. These surgical procedures require verysmall skin and soft tissue openings, often only a few centimeters indiameter. In general, minimally invasive surgeries utilize a specialretractor system that creates a port to the surgical field. Suchretractor systems are typically 18-25 mm wide and 40-110 mm long,however, as the average size of an American person is growing, longerretractors are often needed.

The smaller incisions afforded by these methods have numerous benefitsfor the patient including smaller scars, less blood loss, shorterhospital stays, and faster recovery periods (Sasani et. al., 2010). Theprocedures also reduce trauma, lessen the potential for blood clots, anddecrease costs associated with extended therapy (Rosenthal et. al.,1994).

Given the foregoing, there remains a critical need in the art for aspecific, minimally invasive applicator device

SUMMARY OF THE INVENTION

The present invention embraces the design, construction, and testing ofan applicator device capable of being used in minimally invasiveprocedures. The device or apparatus of the present invention is uniquelydesigned to deliver or administer therapeutic or surgical materials,preferably at an elevated temperature. Such materials include, forexample, sealants, adhesives, therapeutic compositions such as creams,ointments and gels, structural components such as bone, hydroxyapatiteand/or bone wax, and/or particularly malleable materials and thosehaving temperature dependent phase changes.

As used herein, an “elevated temperature” refers to any temperatureabove a baseline temperature. It is contemplated that even roomtemperature could comprise an elevated temperature over the colderenvironment of a sterile surgical field which is below typical roomtemperature of approximately 25° C. Elevated temperature therefore mayrange from 20 to 50° C.

While the apparatus of the present invention may be employed to deliverany material or surgical substance (including therapeutic compounds andcompositions) the apparatus will be described in detail in the contextof use as an applicator for bone wax in the management of hemostasis.

The device of the present invention comprises four interconnectedcomponents or systems. These are the gripping means, the applicatorsystem or tip, the wax loading system or mechanism, and a heatingmechanism. Through an iterative design process, coupled withexperimental testing, the device of the present invention comprises aspring-loaded hook mechanism for actuation of the device, a clovershaped tip with a slight upwards angle for the tip design, a positivethermal coefficient (PTC) heating element wrapped around the waxdelivery tube (bayonet) for a heating element design, and an open slotfor the wax loading. Each of these components and their equivalents isillustrated and described herein in detail.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a one embodiment of the bone wax applicator of thepresent invention.

FIG. 2 illustrates the various component systems of the bone waxapplicator. Shown in general detail are the applicator body, the grip ortrigger assembly, the heating assembly, the wax extrusion rod, the heatsource assembly, and the tip surface.

FIG. 3 illustrates details of the applicator body of the bone waxapplicator of the invention.

FIG. 4 illustrates further details of the applicator body.

FIG. 5 illustrates the front grip trigger assembly.

FIG. 6 illustrates alternate embodiments of the bone wax applicator,with and without tension means.

FIG. 7 illustrates one tip design of the present invention.

FIG. 8 illustrates alternate cavity embodiments for a smooth roundedsurface tip.

FIG. 9 illustrates six additional tip designs of the invention.

FIG. 10 shows images of photographs of three different tip designs whichwere manufactured.

FIG. 11 illustrates the hatch chamber closing mechanism for the waxloading chamber.

FIG. 12 illustrates alternative wax loading chamber enclosures.

FIG. 13 illustrates alternate placement of the heating element of theinvention.

FIG. 14 shows images of photographs of a prototype of the presentinvention, showing the entire bone wax applicator, the tip and theapplicator body and trigger means.

FIG. 15 shows the temperature profiles of various wire compositions usedas heating elements.

FIG. 16 shows the temperature of the wax and bayonet on heating.

FIG. 17 shows graphs of the study of clear and reflective MYLAR asinsulation to thermofoil heating elements.

FIG. 18 shows PTC testing done with different batteries.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a minimally invasive applicator devicefor use in surgical settings. The applicator may be used to applytherapeutic or surgical materials, preferably at an elevatedtemperature. Such materials include, for example, sealants, adhesives,therapeutic compositions such as creams, ointments and gels, structuralcomponents such as bone, hydroxyapatite and/or bone wax, particularlymalleable materials and those having temperature dependent phasechanges.

The bone wax applicator device or apparatus of the present inventioncomprises at least an applicator body, a grip or trigger assembly, aheating assembly, and an extrusion rod, or wax extrusion rod in theexemplary embodiment described herein.

The grip or trigger assembly is an ergonomically friendly, squeezablehandle which can be held with a single hand. Squeezing the handleratchets the wax extrusion rod down the barrel of the bayonet, extrudingthe wax through the tip in a controllable and precise fashion. Thebayonet features a heating element which warms the wax to optimizesurgical application. The heating element is powered by a battery,although other power sources may be used. The heating assembly featuresan insulating layer outside the heating element to keep the outertemperature of the bayonet cool. The grip design also provides for amechanical advantage, so that even if the heating element is turned off,the wax can be readily extruded from the device with a comfortableamount of force. The wax is loaded in the open slot of the top of thegrip, also called the wax loading chamber. According to the presentinvention, the apparatus may be manufactured with the wax or othersurgical material pre-loaded or feature alternative loading portsdescribed herein.

The bayonet is long and narrow with an external diameter ofapproximately 8 mm to allow it to easily fit down a minimally invasivesurgical port as well as optimize the surgeon's field of vision.

According to the present invention, the bayonet may range from 10 to 30cm in length.

The tip attached to, or formed at the end of, the bayonet is preferablyrounded to aid in the spreading of wax across surfaces of cut bone.

According to the present invention, the central cavity of the bayonethas an internal diameter of between 0.1 cm and 1.0 cm and externaldiameter of between 0.5 and 1.5 cm. The internal diameter is preferablyno larger than 8 mm in diameter. The present device allows for theapplication of bone wax to a depth of at least 10 cm to more than 30 cm.The device also allows for controlled quantities of bone wax to bedelivered quickly and precisely to a variety of bone surfaceorientations, and with enough pressure to reliably achieve hemostasis.

Turning to the figures, FIG. 1 illustrates a one embodiment of the bonewax applicator 100 of the present invention. The device is shown withthe extrusion rod engaged and having an angled tip.

FIG. 2 illustrates the component systems of the bone wax applicator 100.Shown in general detail are the applicator body 10, the grip or triggerassembly 20, the heating assembly 30, the wax extrusion rod 40, the heatsource assembly 50, and the tip surface 60. Each of these componentswill now be described in detail.

Turning to FIG. 3, the applicator body 10 of the bone wax applicator ofthe invention 100 comprises the rear grip 11 (further detailed in FIG.4), a wax delivery tube or bayonet 15 through which the bone wax travelsfor delivery to the site of administration, a tip 16, a wax loadingchamber 17 shown in the figure with an open hatch design for receivingthe bone wax and a battery housing 18.

The applicator body including the rear grip, bayonet, tip, wax loadingchamber and battery housing may be machined, made, formed, molded ormanufactured as one piece or in components which are assembled in aconfiguration as shown. For example, the battery housing may bemanufactured separately from the applicator body and then attached orconnected to the applicator body.

It is contemplated by the present invention that various types of tipsmay be employed in conjunction with the bone wax applicator and as suchvarying tip designs (discussed herein) may be made to be affixed, eithertemporarily or permanently, to the terminus of the bayonet. Tips mayalso be an integral part of the applicator body and therefore be made aspart of the “one piece” design.

The length of the tip may vary from 0.5 cm to 5 cm. Tips may bestraight, angled or curved.

In functioning, bone wax is inserted into the bone wax chamber 17 andpushed through the bayonet by the forward actuation of the wax extrusionrod 40. The wax extrusion rod comprises a first region, a second regionand optionally a third region. The proximal end of the wax extrusion rodor first region represents the plunger region 41. It is this regionwhich is inserted into the rear opening of the applicator body 10, andtravels into and through the wax loading chamber 17 and inside thebayonet. This first region is connected immediately distal to a secondregion, the notched region 42 of the wax extrusion rod. The notchedregion is designed to engage a spring loaded hook 21 which is actuatedby the movement of the front grip trigger assembly 20 (detailed in FIG.5). The distal portion or optional third region of the wax extrusion rod40 is the handle region 43. The handle region is utilized by theoperator of the applicator to align or position the rod into theapplicator body. Although not shown in the figure, the extrusion rod maycontain a small hole passing perpendicularly and cross-wise throughregion three through which a ring or bar may be threaded to serve as ahandle so the extrusion rod can be more easily retracted.

Both the applicator body and wax extrusion rod may be manufactured fromaluminum, stainless steel or other suitable material.

Once wax has been inserted into the wax chamber 17 and the wax extrusionrod 40 inserted into the applicator body, the trigger assembly 20 isengaged by the operator by pulling the grips together using a“squeezing” motion. The actuation of the spring loaded hook 21 about thegrip attachment means 22 against the notched region of the extrusion rod42 forces the bone wax into the bayonet 15 and out the tip 16. Withinthe bayonet, the wax is heated. Heating is achieved along a portion ofthe bayonet by the heating assembly.

The heating assembly comprises a heating element 31, which is operablyconnected to the current source (here, a battery 51). In one embodimentthe heating element is a PTC element comprising conducting ink printedon filament paper. Other heating elements may be used and includethermofoils, wires and the like.

The connection between the heating element and the source of current maycomprise any electrically conducting material. In one embodiment theheating element connector 32 comprises electrically conducting lines ofink printed on filament paper. Connection between the heating elementand the current source may be made via any type of connector such as awire, conduit or metal strip, filament or string. Shown in the figure isa battery connector 52 operably connecting the battery 51 to the heatingelement 31 via a heating element connector 32. The battery connector 52is operably linked to the battery via a battery connector means 53(shown in FIG. 5). An opening in battery housing 19 is used forreceiving the battery connector. It is not necessary that the heatingassembly cover the entire length of the bayonet.

In one embodiment, the heating assembly covers at least 40 percent ofthe bayonet. The heating assembly may be designed to cover from between30-60 percent, between 20-80 percent or between 10-95 percent of thelength of the bayonet.

For optimal wax heating, it is important to control the temperaturealong the bayonet. Insulating means 33 may be used to achieve thisobjective. Insulation around or along the bayonet may comprise a solidmolded material applied to the surface of the heating mechanism orelement or may be wrapped around the outside. Insulation may comprise asingle layer or multiple layers. In one embodiment, the insulation 33 iscomprised of plastic, plastic film or sheet, MYLAR™.

In a further embodiment, the insulation may be covered with one or morelayers of heat shrink tubing 34. This tubing may be made of fluorinatedethylene propylene (FEP) heat shrink tubing or other suitable material.

FIG. 4 shows further details of the applicator body. Above the rear grip11, is a rear grip guard 12. This guard serves to stabilize the devicein the hand of the operator. The rear grip also contains a passage 13through which extends a lever for disengaging the spring loaded hook andconsequently the wax extrusion rod from the device.

Atop the applicator body is a mounting platform 14. This platform may beused to attach (either temporarily or permanently) an alignment device,sight, camera, mirror, fiber optics, laser, scope or other optical toolto aid in the alignment or positioning of the bone wax applicator. Theplatform may also serve as an attachment point for a surgicalinstrument, light, handle, clip or any device that may be useful inconnection with the utilization of the bone wax applicator.

The actuation of the notched region 42 of the wax extrusion rod throughthe wax loading chamber and into the cavity of the bayonet isillustrated in FIG. 5. Here, a spring loaded hook 21 centrallypositioned in the shaft of the front grip and supported by a spring 24engages the notched region in a ratchet type mechanism forcing the waxextrusion rod forward.

A groove 23 in the spring loaded hook allows the spring loaded hook toslide along the longitudinal axis of the front grip about the attachmentmeans. In operation, the hook member 25 of the spring loaded hook isforced against the notched region 42 and may be disengaged by theoperator pressing and holding down the lever 26.

The bone wax applicator may be fitted with tension means 27 attached tothe grips or trigger mechanism by any tension attachment means 28including, but not limited to screws, pins, hooks and the like.Alternate embodiments of the bone wax applicator, with and withouttension means, are shown in FIG. 6.

FIG. 7 illustrates one tip design of the present invention. As shown inthe figure, the tip 16 may be configured or manufactured to becontiguous with the bayonet. The tip may represent at least 2, at least3, at least 5, at least 10, at least 25, at least 30, at least 40 or atleast 50 percent of the length of the bayonet.

Tips may be made as variable attachments to the bayonet.

Tips may be straight or have an angle of incline from the horizontal asmeasured along the longitudinal axis of the bayonet. The tip shown inFIG. 7 is a contiguous angled tip.

The tip surface 60 shown in FIG. 7 is rounded and smooth. In alternateembodiments the tip surface may be of any shape including square,triangle, or any polygonal shape with any number of sides or facets.According to the present invention, the tip cavity 61, may have aninternal diameter equal to or smaller than the bayonet. The cavity mayterminate in one or more holes or ports and may be of any shape, bevelor size relative to other ports or the bayonet internal diameter. Shownin FIG. 7 is a single port tip with a clover leaf cavity havinginternally beveled edges.

FIG. 8 illustrates alternate cavity embodiments for a smooth roundedsurface tip. FIG. 8A shows a side view of a straight tip attachment witha rounded smooth surface. FIG. 8B shows the end view of three cavityconfigurations, two multi-cavity and one single cavity design. It isshown that multi-cavity designs may include overlapping cavities ordistinct cavities.

FIG. 9 illustrates six additional tip designs of the invention. FIG. 9Ashows a straight tip with a rounded surface and smooth cavity exit. FIG.9B shows a straight tip with a beveled cavity exit. FIG. 9C shows astraight tip with a triangular notched cavity exit. FIG. 9D shows astraight tip with a square notched cavity exit. FIG. 9E shows a straighttip with a straight lip (spatula) cavity exit. FIG. 9F shows a straighttip with an angled lip (spatula) cavity exit.

FIG. 10 shows images of photographs of three different tip designs whichwere manufactured. FIG. 10A shows an angled tip with a smooth surfaceand three exit cavities. FIG. 10B shows a straight tip with a smoothsurface and one clover leaf exit cavity. FIG. 10C shows an angled tipwith a flattened exit cavity. The angle of the tip is 10 degrees. Theangle of the tips of the invention can individually range from 5-90degrees. In some instances, it may also be desired to have an angle ofgreater than 90 degrees, for example 90-120 degrees in order to applybone wax to an obscured surface.

When manufactured as an attachment, tips may have rotating heads,multiple heads or be configured as a separate device with differentheads, surfaces or cavity. The tips may be manufactured of material thatdiffers from that of the bayonet and as such may be bendable orindependently positionable. A multi-head or tip design would allow forthe application of bone wax to more than one surface at a time.

FIG. 11 illustrates the hatch chamber closing mechanism for the waxloading chamber. In this embodiment, the wax loading chamber isconfigured with a hatch cover which can lock into place as shown. Thisconfiguration is advantageous in that it reduces the exposure of the waxto the surgical field or external environment.

Other wax loading chamber enclosures are shown in FIG. 12. Here, asliding hatch and a rotating hatch are shown.

FIG. 13 illustrates alternate placement of the heating element of theinvention. FIG. 13A illustrates a grip hearing element while FIG. 13Billustrates a tube (bayonet) heating element.

FIG. 14 shows images of photographs of a prototype of the presentinvention, showing the entire bone wax applicator (FIG. 14A), the tip(FIG. 14B) and the applicator body and trigger means (FIG. 14C).

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the bone wax applicator featured in theinvention, suitable methods and materials are described below.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Incase of conflict, the present specification, including definitions, willcontrol. In addition, the materials, methods, and examples areillustrative only and not intended to be limiting.

EXAMPLES Example 1 Applicator Design

Initial design efforts were directed to optimizing the application ofthe bone wax. To this end, three designs were tested. These included aretractable applicator, a modular applicator, and a roller applicator.

The retractable applicator investigated involved application of wax tothe tip prior to application. In this embodiment, the tip is retractedinto the outer shell and the applicator is inserted into the surgicalsite. The surgeon exposes the tip by pressing the outer button, and thenapplies the wax by depressing the inner button.

In the modular applicator, the grip and the tip designs are separated.Bone wax is added to the device by manually placing a small amountinside the tip before connecting it to the grip. Multiple tip designssuch as those of FIG. 8 could be used.

While only three tip designs are shown, this modular design could allowfor numerous other tip design alternatives. In particular, one tipdesign alternative could allow for bone wax to exit through a side port,and another could apply wax through a vertical port. While themodularity of the tips is an advantage of this design, attaching theproper tip may prove cumbersome and aggravating in the operating roomenvironment.

The roller applicator design alternative uses a syringe to push wax ontoa roller. As the handle is compressed, the flexible bar attaches tonotches in the syringe and pushes bone wax down the device shaft. Whenthe handle is released, the metal bar releases from the syringe andreturns to the resting position. The surgeon then could use the rollerto apply an even layer of bone wax to a desired bone surface.

Although novel in design, the roller applicator has many design flaws.The roller tip requires lateral movement that the small incision doesnot afford. Furthermore, the adhesive properties of the bone wax maycause rotation problems with the roller. Due to these limitations, thisdesign alternative was abandoned.

A common theme among the preliminary design alternatives was thesyringe-like mechanism. One concern was that unheated wax would be toostiff to squeeze out of a syringe outlet with an outer diameter of 8 mmor less. To investigate this concern, tests were performed using bonewax and two syringes, each with a 4.5 mm diameter outlet. The twosyringes had an inner diameter of 15.6 mm and 14.4 mm, respectively. Thebone wax was handled as minimally as possible with gloved hands torefrain from heating the bone wax via body warmth. The bone wax wasplaced in each syringe and the relative difficulty of squeezing out thebone wax was assessed.

Test results revealed that wax was more easily expelled from the 14.4 mmsyringe. It was hypothesized that this was due to the smaller differencebetween the syringe diameter and outlet diameter. The room temperaturebone wax proved to be difficult to squeeze out of the syringe with onehand. The tests were repeated after the bone wax was warmed with agloved hand, which greatly increased the ease with which the bone waxcould be squeezed out of the syringes.

The tests strongly suggested that the syringe design alternatives shouldprovide a method of giving users a mechanical advantage in expelling thebone wax. The results also suggested that the bone wax applicator shouldretain a similarly sized outlet as the inner diameter to ease waxextrusion.

A proof of concept test was then performed to determine if a mechanicaladvantage could be sufficient to extrude unheated wax. A lever arm wasused to push a syringe plunger through a straight tube approximatelyeight inches in length.

As the syringe tube inner diameter and the outlet diameter were the samesize, the wax was easily extruded from the tube. Although the proof ofconcept prototype was unable to conclusively demonstrate the feasibilityof using a mechanical advantage for unheated wax extrusion, there weretwo important results. Firstly, it was found to be nearly impossible tocontrol the wax extrusion when the outlet was the same diameter as thetube, for there was nothing to prevent the wax from falling out of thedevice. The team therefore considered adding a slight taper to the tip,although not so much as to greatly increase the force required for waxextrusion. Secondly, the testing revealed that it took a long time toprepare and reshape the bone wax so that it could be loaded into the endof the tube. As the final bone wax applicator needed to improve the easeof applying bone wax, it was decided to consider methods of adding waxinto the device. Such wax loading designs are described in Example 4.

As the previous test was inadequate in determining if a mechanicaladvantage would be sufficient in extruding bone wax, a more rigoroustest bench was created. The acrylic test bench consisted of a set ofsquare plastic channels, which varied the inlet and outlet sizes, aswell as the taper between the two cross-sections. The dimensions aregiven in Table 1.

TABLE 1 Test bench channel dimensions Inlet Taper Outlet diameter lengthdiameter (mm) (mm) (mm) 6 10 5 6 10 4 6 5 5 6 5 4 6 2 5 6 2 4 5 10 4 510 3 5 5 4 5 5 3 5 2 4 5 2 3

The channels, which were precisely cut using a CNC machine, were cutsquare due to limitations in rapidly prototyping models that includedrounded slopes. However, a circular channel was preferred in the finaldesign to maintain a symmetrical design with no sharp edges. The testbench design required two symmetrical pieces to be clamped togetherduring tests, and is a concession both to manufacturability and ease ofcleaning.

Wax strips of approximately 4 cm in length were placed into thechannels. A square piston that fit into the channel was used to push outthe bone wax. For each test, a constant force was applied to the piston(48 N), and the time needed to extrude one centimeter of unheated bonewax was recorded. Six trials were performed on each of these channels.

The results from the quantitative force tests for the 6 mm inlet portchannels suggested that wax flow rate is independent of taper length.However, flow rate did change with outlet diameter. Tests suggested thata smaller outlet diameter resulted in slower flow. While individualtests fluctuated more than desired, this latter conclusion was expectedand further validated the overall testing. Thus, to attain a larger waxflow rate, the device was designed to have a larger outlet to inletratio, regardless of taper length.

In an effort to determine the necessary mechanical advantage, a blindtest on eight individuals was performed to find the force that could beexerted comfortably. The test suggested that humans could exert anaverage grip clenching force of 15 N. The acrylic tube tests wereconducted with a 48 N constant downward force. Thus, a design thatimplemented a 4:1 mechanical advantage was considered sufficient toextrude bone wax without use of a heating element. From these results,the retractable applicator design was abandoned.

After determining the necessary mechanical advantage for the device,subsequent design efforts focused on four subcomponents: grip designs,tip designs, wax loading designs, and heating element designs. Each arediscussed in the following examples.

Example 2 Grip Design

Five grip design alternatives or trigger assemblies were evaluated andtested.

1. Lever Arm.

The lever arm is the simplest grip design. The top lever pushes aplunger through the tube attached to the bottom lever, extruding thewax. The tube travels along a track cut into the bottom lever so that itremains concentric with the plunger and torque is not experienced as theplunger is pushed down.

A three-dimensional printed model of the lever arm was developed. Thisproof-of-concept was ergonomically awkward and was very difficult tocontrol the tube from which the bone wax was extruded. This suggests itwould be less precise in applying bone wax to a specific location.

One advantage of this design is that it can be modified for thenecessary mechanical advantage, which is based on the ratio between thelever length and the distance from the plunger to the hinge. Themechanical advantage translates a large force to a small distance.Furthermore, the plunger can only move a set distance; meaning onlysmall amounts of bone wax can be extruded before reloading is necessary.Due to these reasons this design was abandoned.

2. Worm Gear.

The worm gear design works by rotating a side-mounted lever. Angledgears cause the central worm gear to rotate in sync with the lever arm.The rotation of the worm gear pushes the cylindrical plunger downward,forcing the wax out to the tip.

To assess the user interface of the design, a three-dimensional modelwas printed. Although the model did not have the inner gears, itprovided insight into other aspects of the design. Firstly, the designcannot be used with one hand because it requires one hand to stabilizethe device while the other hand rotates the hand crank. Secondly, inorder to get a sufficient mechanical advantage, the angled gears must beseveral centimeters in diameter, which blocks the surgeon's view. Theinternal mechanism also has numerous moving parts that make it moresusceptible to jamming and manufacturing defects than simpler designs,thereby making it less reliable. Due to these disadvantages, the teamabandoned this design.

3. Ratchet Gear.

This design uses a ratcheting mechanism to extrude bone wax. A bendingstop-pin next to the circular gear allows the gear to rotate in onedirection, but prevents the gear from rotating backwards. The usersqueezes the handle, which rotates the gear with a push-pin located onthe pivoting handle piece. The rotation of the gear moves the plungerdownward to extrude the wax. A compression spring (not shown) betweenthe two handle pieces forces the pivoting handle section to revert backto its starting position. By design, the push-pin bends slightly as thehandle returns to its fully open position, allowing the lever handle tomove while the gear remains stationary.

This design provides a mechanical advantage that depends on the size ofthe circular gear and where the force is applied on the handle. Thecurrent design only allows for a single use, as the stop-pin preventsthe plunger from being pulled back to its original position; however,modifications are possible. A three-dimensional printed model of thedesign was created. The initial printing suggested that push-pin and thestop-pin were too small and weak to adequately turn or stop the gearrotation, and might cause the device to be unreliable. However, theoverall design was ergonomically similar to medical devices already usedduring surgeries, and preliminary testing demonstrated that it was botheasy to use, and easy to precisely direct where the tip was pointing.

4. Caulking Gun.

Instead of a gear, the caulking gun design uses a friction-springmechanism to grip the plunger and slide it down the tube. Aproof-of-concept model was constructed using the grip of an actualcaulking gun with an aluminum tube attached. One advantage of the designis that the plunger is retractable, and therefore allows for the deviceto be refilled.

The mechanical advantage of this design is the ratio of the length ofthe bottom handle to the distance between the two screws on the lowerhandle. While the proof-of-concept was large and bulky, subsequentiterations of this design were streamlined and lighter to improvevisibility for the surgeon. The caulking gun design was more robust andreliable than the other grip designs, and was also easy to use.

5. Spring Loaded Hook.

The spring-loaded hook design was created in response to faults in thecaulking design. See FIG. 5. It works in conjunction with a notched rodto extrude bone wax. The design uses a spring loaded hook having a hookmember that slides in and out of the trigger assembly. A spring incompression between these two pieces ensures contact between the hookmember and the notched extrusion rod. When the handle is released, aspring between the two handle pieces forces the handle pieces apart. Theshape of the hook member, along with the notch design, allows for thespring loaded hook to compress the spring slightly, allowing it to glideover the rod without pulling it backwards. There are significantadvantages to this design. Firstly, there are few parts compared toprior examples. Secondly, most of the design is internal. Thirdly, it isrelatively easy to incorporate a release mechanism into the design. Bypushing down on the lever, the connection between the rod and the hookmember of the spring loaded hook is disengaged, which allows for theextrusion rod to be retracted. Due to the force on the lever, theselected material is required to be very strong, such as stainlesssteel.

Example 3 Tip Design

The tip is the outlet of the bone wax applicator that shapes and spreadsthe wax. It plays a key role in controlling the wax application.Numerous tip designs were evaluated throughout the design process. Theywere divided into four categories: regular tip, modified tube tip,spatula tip, and mushroom tip.

1. Regular Tip.

The regular tip is the simplest tip in design. It is essentially anextension of the tube, but with a slightly smaller diameter. The smallerdiameter is meant to compress and shape the wax prior to application.Although the simplicity of this design is appealing, it does not providea large surface to spread the wax onto the bone. One example is given inFIG. 9A.

2. Modified Tube Tip.

Three modified tube tip alternatives were investigated, all of which arealtered regular tips. The one-notched tip, FIG. 9C, consists of aregular tip with a small lip that can be used to spread the wax.Similarly, the two-notched tip, FIG. 9D, has two lips that can shape thewax. The slanted tip, FIG. 9B, is composed of a regular tip with anangled cut. All of these modifications were meant to facilitate inshaping and spreading the wax. However, a disadvantage of these designsis that they are unfamiliar to the user, and they are unidirectional,meaning the device requires appropriate orientation in order to spreadthe wax.

3. Spatula Tip.

The spatula tip design is similar to the tip of a Penfield applicator.The user extrudes the wax from the tube onto the tip, and can use theslanted surface of the tip to scrape or press the wax onto the bone.This design would be used similarly to the current procedures, and wouldbe familiar to new users. The primary disadvantage is that the tipdesign is unidirectional and requires appropriate orientation to applythe wax in the desired direction, thus it is more difficult to use thanother tip designs. It does, however, allow bone wax to be applied tomultiple surface angles, as the spatula would facilitate application toside surfaces. Spatula tips of the present invention are shown in FIGS.9E and 9F.

4. Mushroom Tip.

The mushroom tip design consists of a symmetrical convex plate with anexit port for the wax at the center. See FIG. 8. Although the tube hasan outer diameter of 7 mm, the convex plate is 10 mm in diameter. Theextra area on the tip maintains a large outlet port and provides alarger surface to apply the bone wax. The mushroom design can havevarying numbers of exit ports or openings. See FIG. 8B. Several possibleconfigurations of exit ports were designed—only one exit port, fourdiscrete exit ports, and four exit ports conjoined together (like aclover). Having multiple exit ports that are smaller in size is moredesirable, so that the wax is extruded across a larger surface area ofthe tip and can be applied more easily. Unlike the spatula tip, this tipis not unidirectional, so the wax may be applied in any direction.

Example 4 Wax Loading Design

The method of loading the bone wax into the device is a crucialcomponent of the overall design. It is possible that the finalmanufactured device will be preloaded with wax. However, a method foradding bone wax to a stand-alone applicator would be desired for maximumflexibility. A design that does not allow bone wax to be added easilywill be bothersome and aggravating, and may discourage surgeons fromusing the device. Six wax loading design alternatives were evaluated.

1. Bone Wax Press.

The bone wax press design presses a packet of wax into the extrusiontube of the device. The bone wax press design is limited in width by thesize of the tube's inner diameter; however, the length can be as long asdesired. Since the thickness of the bone wax is initially 5 mm thick,the design interfaces with the initial bone wax sample, so long as thelength of the press opening is greater than the bone wax sample length.As the press significantly protrudes from the side, it cannot be locatedon the part of the device that enters the incision. While the currentprotruding design limits visibility for users, it could be relocated ona different side to increase visibility.

2. Open Hatch.

A more basic loading system would have an open hatch in the tubecreating a chamber in which the wax may be inserted. The hatch would belocated near the top. The wax would be cut to a size that fits thediameter of the tube. After the wax is inserted into the hatch, theextrusion rod would be used to push the wax toward the tip so that morewax may be added. See FIG. 12A.

3. Snap-On Hatch.

A second wax loading system uses a snap-on hatch on the side of thebayonet or tube. The wax is rolled (or otherwise shaped into anappropriate size) and placed into the opening. The hatch is then snappedinto place to close the gap. The snapping mechanism is entirelycontained within the walls of the tube to prevent disturbing the flow ofthe wax inside the tube, and to refrain from blocking the surgeon's viewwith protruding parts. In addition, the snapping mechanism is designedto prevent removal of the hatch after it has been locked in place. Thisensures that the hatch will not fall off during wax extrusion, andenforces the disposability of the device. The materials for the designmust be carefully selected, as the hooks are required to bend slightly,but not break off. See FIG. 11A.

4. Hinged Hatch.

The hinged hatch design allows users to place a rolled piece of wax intoan opening on the side of the tube. This design employs a hinge to keepthe hatch attached to the body of the device to prevent the cover frombeing dropped or lost. Despite the small size of the hinge, itsprotrusion beyond the tube may limit the surgeon's field of vision. Inaddition, the actual size of the hinge may be too difficult tomanufacture. See FIG. 11B.

5. Sliding Hatch.

In the sliding hatch design, part of the tube slides downward andexposes the inner tube for wax loading. It has no protruding parts andall of the parts stay connected. A disadvantage of this mechanism isthat the wax may extrude into the crevices between the two parts of thetube if not designed properly. See FIGS. 12B and 12C.

6. Rotating Hatch.

The rotating hatch functions similarly to the sliding hatch, except thatit has an outer casing that rotates to expose the inner part of thetube. A disadvantage of this design is that it requires a largerdiameter for the outer casing. The hatch could be placed toward the topof the grip, but then the wax would need to be pushed down a longerdistance to reach the tip of the tube for application. Alternatively,the rotating hatch could be placed internally rather than externally,which would be similar to other medical equipment used. See FIGS. 12Dand 12E.

Conclusion.

The open hatch was deemed most desirable for initial prototype purposesas it does not require any moving parts, does not add any width to theoverall device, and is simple and intuitive. An alternative is therotating hatch, but since the wax significantly protrudes from thedevice, it makes the device bulky and impedes the visibility of thesurgeon. The snap-on hatch, which does not allow for wax to be reloaded,would be acceptable as long as a significant amount of bone wax could beadded in the initial loading process. The hinged hatch requires verysmall parts and the protruding hinge may impede surgeon visibility.While the current prototype uses the open hatch, future research andtesting on the wax loading mechanism is recommended. Should the designbe disposable, the optimal device would be preloaded with bone wax.

Example 5 Heating Element Design

Although a heating element was not necessary for a functional prototype,initial prototype testing demonstrated that the properties of heated waxwere preferable to unheated wax. Heated wax spreads more easily and wasmuch easier to apply during testing. The following sections detail thedesign process for the heating element.

1. Wax Temperature Testing.

Testing was done to determine the ideal temperature range to heat thewax. Bone wax was set in a small beaker. The beaker was placed in awater bath. Using a hot plate, the wax was heated and the properties ofthe wax were observed as it heated up.

The qualitative properties of the wax were felt by hand as the waxheated up. Table 2 details the quality of the wax at varyingtemperatures.

TABLE 2 Temperature v. Quality Wax Temperature (° C.) Quality <30 Hard30-35 Starts to soften 35-40 Easy to spread with fingers 40-50 Very easyto spread with fingers >50 Begins to melt

From this testing, it was decided to heat the wax to 45° C., as that wasthe median in the temperature range for wax of very good consistency.However, the introduction of a heating element presented newconstraints. The outer temperature of the tube would ideally remain asclose to 37° C. (body temperature) as possible, so as not to harm thepatient or the doctors and surgical technicians handling the device.Thus, insulation must also be considered. The heating element shouldlast at least one hour, which is approximately the length of time thedoctor may need to use the applicator in a given surgical procedure. Thedevice should be powered through a battery so as not to clutter theoperating area with a power cord. Lastly, the device should warm the waxquickly.

2. Heat Source Locations.

Another design consideration for the heating element was where theelement should be located on the device. Two possible locations for theheating element were explored—at the trigger assembly (grip) or aroundthe bayonet.

A heating element at the grip would rely on the conductivity of thematerial of the tube to transfer the heat to the wax inside. Containingthe heating element within the grip does not influence how wax can beloaded into the device, and it maintains most of the dimensions of thedevice, thus requiring minimal adjustments to implement. However, havingthe element at the grip means that the element will have to reach highertemperatures in order to warm the wax, and insulating the heatingelement at hotter temperatures would be a larger challenge.

Alternatively, a heating element around the bayonet is advantageous inthat it does not have to reach a temperature much higher than 45° C.However, such a heating element in addition to insulation adds materialto the bayonet, and thus to maintain an outer diameter of approximately8 mm, the inner diameter will have to be smaller, decreasing the amountof wax that can be stored in the bayonet. To retain the small diameterof the bayonet, the heating element needs to be very thin and flexibleso that it can wrap around the tube.

Example 6 Heating Element Location, Size, Material and Insulation

Rather than immediately ordering and testing many different heatingelements at various locations and sizes, a finite element model analysiswas performed using COMSOL. This tool allowed estimation of the heatingbehavior of different heating elements before further pursuing aspecific design implementation.

Four categories of the design were selected for further testing afterthe modeling: location of the heating element, size of the heatingelement, material for the tube, and insulation. All of the COMSOL modelsassumed a heating surface of 45° C., 15 cm of bone wax loaded from thetip end, and a 6 mm ID and 7 mm OD tube. The models were analyzedparametrically in time up until 30 minutes. 2D Axial Symmetry wasassumed and the standard Heat Transfer Module for COMSOL was used.

1. Location of the Heating Element.

As previously mentioned, the heating element for the caulking gun designcould be located in two places: near the trigger assembly (grip) oraround the bayonet. The grip heating approach was designed such thatonly the circular base of the bayonet in direct contact with the grip(excluding the threads) was heated. Results showed that aluminumperformed best. However, even after 30 minutes, the bone wax did notreach 45° C., which was the desired temperature for optimalmalleability.

The heating element was then moved to the outside circumference of thebayonet. A 1 cm heating strip that wrapped around the bayonet was drawnaround the midpoint of the wax in the bayonet. The outer boundary of theheating element assumed perfect insulation, meaning this modelingrepresented the best-case scenario for the heating element. Results forheating around the bayonet showed that almost all of the wax reached 45°C. in the aluminum model. Thus, it was concluded that heating around thebayonet would be more effective.

2. Size of the Heating Element.

Because the entire bayonet would contain wax, a larger heating stripcould be employed to heat along the entire bayonet. To explore thisoption, a model was drawn such that the entire 15 cm of bayonet thatcontained the wax was encased in a strip of heating material. As withthe 1 cm heating strip, this modeling assumed perfect insulation.Results showed that a heat strip that covered the entirety of thebayonet would be ideal.

3. Bayonet Material.

The previous models used various materials for the bayonet: acrylic,DELRIN™, stainless steel, and aluminum. It was concluded that acrylicand DELRIN™ should not be used, as they act as insulators and resistheat transfer. However, the stainless steel and aluminum modelsfacilitated heat transfer.

4. Insulation.

To prevent the higher temperature of the bayonet and heating elementfrom coming directly into contact with the patient's tissues, a layer ofinsulation was needed. Three materials were investigated for their lowthermal conductivity polyimide (e.g., KAPTON™), MYLAR™, and STYROFOAM™.

A 1 mm thick layer of insulation was drawn around the entire bayonet,which encased the bayonet and the 15 cm heating strip around thebayonet. All three insulators had very similar results and none coulddecrease the outside temperature to the body temperature of 37° C.However, as thinner STYROFOAM™ would be difficult to handle, onlyKAPTON™ and MYLAR™ were explored further.

Example 7 Heating Methods

Having modeled the heating element with COMSOL, it was necessary todetermine different methods of implementing the heating element. Detailsas well as advantages and disadvantages of each method are describedherein.

1. External Heating Accessory.

One heating method would be to use an external accessory heating device.When the bone wax applicator is not in use, it is placed on a stand orin a holster that warms the device and the wax. This heating elementwould require minimal alterations to the design of the applicator.However, the material used to construct the applicator is still animportant consideration. While a viable option, an accessory devicedesign was not further optimized.

2. Wire Element.

An alternative way to heat the wax is to use a wire heating element. Thewire may travel straight down the bayonet or it may coil around thebayonet circumference. It was hypothesized that a 9 V battery should besufficient to power the wire and provide enough heat to keep the waxmalleable without melting it. The material used to construct theapplicator must be carefully considered, for if the applicator is notproperly insulated the heat will be transferred to the environment orthe patient rather than to the wax.

Testing was done to assess the feasibility of using a wire to heat thewax. Two high resistance wires were evaluated: a NiCr alloy wire and aFeCrAl alloy wire. For the test, 2 ft of wire was cut and coiled arounda steel bayonet that contained wax inside. The ends of the wire wereconnected to a power source. A thermocouple probe measured thetemperature of the wax in the bayonet. The goal of the wire testing wasto measure the temperature over time at different voltages. However,upon testing, it was discovered that the wires could only handle alimited amount of voltage. The temperature profiles are shown in FIG.15. The most promising wire was the FeCrAl wire at 1.4 V. With thatwire, the wax reached a temperature of 40° C. within 30 minutes.However, this heat up time was slow compared to that of the thermofoiland positive thermal coefficient (each discussed below).

3. Chemical Heating.

Chemical compounds are another way to heat the bone wax. There are twoprimary methods to generate heat chemically. One is with a solution ofpowdered iron, sodium chloride, and charcoal, which heats up viaair-activation. Another method is using sodium acetate, which requires ametallic trigger to initiate solidification and release heat.

To test the air-activated chemical solution, a commercially availableheat pack was tested. The test sought to determine what temperature thechemical solution can reach, but did not test how long the heat lasts.The heatwrap was cut so that a thermocouple probe could be placed insidethe solution and the temperature was measured.

Although the chemical powder did reach over 60° C. within 10 minutes,this design was not further pursued as other alternatives proved morepromising.

The best scenario for implementing this method would be in a chemicalsleeve, but heating accessories are not ideal in an operating room.Implementing within the device would be difficult because it would haveto be enclosed in an airtight container, and then some mechanism forexposing it to air to activate the heat would be necessary. Enclosingthe chemical within the device would be a major challenge in themanufacturing process. Also, it is unknown if chemical exposure tointernal tissue is harmful.

Sodium acetate from a CVS/pharmacy Neck Heat Pack was also tested. 90 mLof solution from the heat pack was poured into a cup, and then ametallic trigger caused the solution to heat up. The sodium acetate alsoheats up quickly, and remains above 45° C. over the course of an hour.

Like the iron solution, the sodium acetate solution would also bedifficult to implement. With a bayonet that is only 8 mm in diameter,adding a chemical layer is not practical. Additionally, when sodiumacetate comes into contact with skin, it causes irritation. This couldbe a problem should the bayonet crack and this chemical solution entersthe surgical wound of the patient. In general, the chemical heatingmethods are a possible alternative for the device but were initiallydeemed less promising than other alternatives.

4. Thermofoil.

A method similar to a wire element is a thermofoil heating element.Thermofoils are resistive wires that have been flattened to increase thesurface area of the wires. They are encased in thin, MYLAR™ sheets, andare very flexible, which allows them to be wrapped around the smalldiameter of the bayonet. The thermofoils vary in resistance and wattageoutput, which in turn varies the overall temperature. The thermofoilrequires a temperature sensor and a control system to ensure that thetemperature does not exceed the temperature constraint. This adds to thecost and complexity of the overall heating element.

Thermofoils, a control system, and thermistor and RTD sensors werepurchased to evaluate this heating option. The thermofoils with higherwattage output per area density output the most heat, as expected. Itwas found that a wattage density of 5 W/in̂2 was the minimum requiredoutput to ensure that the heating element could reach 45° C. at 9V.Higher wattage density heaters had faster heat up times.

As the thermofoil is not self regulated, it therefore requires a sensorand control system to set the temperature. Both the resistancetemperature detector (RTD) sensor and the thermistor sensor are twooptions for temperature sensors. As the RTD sensor is 0.7 mm thick, hasa fast reaction time to temperature changes, and is nearly linear withtemperature in the 20-45° C. range, it was considered the optimaltemperature sensor for this heating design. The implemented RTD sensorwas the S651PDY24A RTD sensor created by Minco®.

Bang-bang (on-off), proportional, and PID(Proportional/Integral/Derivative) controllers were evaluated. Since itwas not problematic if the temperature varied slightly (±1° C.), andsince the bang-bang controller was the simplest design, this type ofcontroller was chosen to control the thermofoil temperature. Forpreliminary tests, the CT-325 bang-bang controller, created by Minco®(Minneapolis, Minn.), was used. While this particular controller isabout the size of a 9V battery, future work would seek to decrease theoverall dimensions of the control system, should the thermofoil beimplemented in the final device.

A third controller system, the CT-198 made by Minco® (Minneapolis,Minn.) works with select thermofoil heaters and does not require atemperature sensor. It instead determines the temperature by sensing apulse of current into the thermofoil, and measures the varyingresistance of the thermofoil, which changes with temperature. While thiscontroller is optimal in that it does not require an additionaltemperature sensor component, it was not tested.

Testing was done on various thermofoils to determine what voltage wasneeded to power the thermofoils to reach the target temperature of 45°C. It was also considered to use thermofoils without a controller.However, as thermofoils are designed to be used with a controller, theywould likely not be FDA approved without a control.

The first thermofoils tested were the KHLV105/2, KHLV105/5, andKHLV105/10, ordered from Omega, where 2, 5, and 10 indicate the wattagedensity per inch squared. The first test done on these thermofoils wasmeant to give an idea of how they behave. The foils were connected to a9V battery and the temperature was measured with a thermocouple untilthey reached 45° C.

From this preliminary thermofoil testing, it was determined that 5 W/in²is the minimum wattage density required to generate enough heat with a9V battery. Also, initial testing with the thermofoils demonstrated amuch faster heat up time compared to that of a coiled wire. The 5 W/in²and 10 W/in² foils heated up to 45° C. within three minutes.

The 5 and 10 W/in² thermofoils were tested in more detail, in whichtemperature was characterized over an hour with different voltages. Fromthese tests, the team concluded that the 5 W thermofoil requires a powersource of 9V to reach the desired temperature, whereas the 10 Wthermofoil requires between 5 and 7 V.

Additional thermofoils by Minco (Minneapolis, Minn.) were also tested.These additional thermofoils were not labeled by their wattage density,but rather by their resistance. Table 3 lists the different resistancefor the thermofoils.

TABLE 3 Thermofoil properties Thermofoil Resistance HK5165 52.3 HK516252.3 HK5164 78.4 HK5160 157 HK5166 529

The Minco thermofoils were tested with a 9V power source and with a 9Vbattery. They were compared with the 5 W thermofoil. Testing suggestedthat lower resistance thermofoils reached higher temperatures. They alsolast over an hour with the 9V battery. Based on these tests, the 5 Wthermofoil was optimal. However, since the thermofoil would likelyrequire a controller, the team determined that a 10 W foil could beused, which would heat up faster than the 5 W thermofoil.

Testing was also done to ensure the functionality of the controller. Thecontroller was tested with the 5 W thermofoil. It was set to maintainthe temperature at 38° C. The test used a 9V battery as the powersource. The controller was very successful at maintaining thetemperature and did not significantly slow down the heat up time.

Lastly, the 10 W thermofoil was tested with the controller and 9Vbattery. The controller was set to maintain a temperature of 41° C. Thethermofoil was wrapped around a stainless steel bayonet with wax inside.Five layers of clear MYLAR™ were wrapped around the thermofoil and theouter temperature of the bayonet was measured. The test showed that the10 W thermofoil would successfully heat the wax and maintain the heatover the course of an hour, and the MYLAR™ would sufficiently insulatethe bayonet. The results are shown in the graph in FIG. 16.

Example 8 Insulation Testing for Thermofoils

Two kinds of MYLAR™ clear and reflective were investigated. Five layersof MYLAR™ were wrapped around a 5 W thermofoil, which in turn waswrapped around the bayonet. The temperature of the thermofoil and MYLAR™were measured. Both types of MYLAR™ kept the outer temperatureapproximately 7 degrees cooler than the thermofoil. The results areshown in the graphs of FIG. 17.

Example 9 Positive Thermal Coefficient (PTC) Heaters

Positive Temperature Coefficient (PTC) heaters have a nonlinearrelationship between their resistance and temperature. As such, when thetemperature increases, so does the resistance. This internal negativefeedback ensures that the temperature of the strip will never exceed afixed temperature for a given input voltage. This allows for a regulatedfinal design without a feedback controller.

PTC elements exist in many forms; PTC thermistors resemble normalresistor, while PTC sheets are similar to thermofoils. PTC sheets can bescreen printed, which allows for quick and inexpensive production ofcustomized heaters. One concern about sheet printed PTC heating elementsis that the printed wires may break if bent too sharply.

Both the PTC thermistors and the PTC sheets were tested. The PTCthermistors were considered as a possible method to heat the wax fromthe grip. However, modeling showed that heating at the grip is lessfeasible due to constrained methods of insulating the heating element.The thermistors were then considered as a possible controller, to beused in series with the thermofoil. The PTC sheets, which were obtainedlater, would be used similarly to the thermofoil, but without the needof a controller.

The team tested two PTC heating elements ordered from Murata (Smyrna,Ga.) at different resistances—one was a 470 ohm resistor, and another a1.8 ohm resistor. Table 4 lists the equilibrium temperature observed atvarying voltages for the resistors.

TABLE 4 Equilibrium Temperatures v. Voltage Resistor Voltage T (C.) 1 ×470 ohm 10 26.7 1 × 470 ohm 15 30.6 1 × 470 ohm 20 32.6 3 × 470 ohm in10 23.9 series 3 × 470 ohm in 20 29.6 series  1 × 1.8 ohm 1 31.7  1 ×1.8 ohm 2 44.4

The 470 ohm resistor was tested individually and with three in series.The 1 Ohm resistor was tested individually. The 470 ohm resistor did notreach the target temperature of 45° C., either individually or in serieswith a voltage input of 20V, and thus would not function for the heatingelement requirements. The 1 Ohm resistor only requires 2V to reach 45°C.

As grip heating would be less practical than heating around the bayonet,the 1 Ohm PTC in series with a thermofoil was investigated. To testthis, a 5 W thermofoil was placed in series with a 1 Ohm PTC and thetemperature was measured after one hour. Based on the results,implementing a PTC in series with a thermofoil could regulate thetemperature, but would require a higher voltage source, so this setup isless desirable.

In a wax transmission test, a 5 mm thick piece of wax (opened directlyfrom the wax packet) was set above a 10 W thermofoil. The temperature ofthe thermofoil and the top of the wax were measured to observe how theheat transfers through the wax. The results indicate that the time foreach to reach 40° C. was about 40 minutes. This testing suggested thatthe heat transfer through the wax must be considered, as it does takesome time to warm all of the wax. However, the wax in the bayonet willhave a diameter of about 5 mm, and as the thermofoil will be wrappedaround the circumference of the bayonet, the heat transfer will takeless time than it did in this test.

The PTC sheets were tested similarly to the thermofoils. For testing,the PTC sheet was wrapped around a stainless steel bayonet, and thetemperature of the wax was measured at 6V, at which point the PTC exceed45° C. The data revealed as steep temperature increase to 45° C. inabout 10 minutes with temperature plateaus at slightly over 50° C. by 20minutes.

The PTC sheet was also tested with a 9V battery to determine whether acapacitor would be required to delay the initial voltage input to thePTC, so as not to ruin the PTC. Testing showed that too much currentdraw at the beginning would not be an issue. The data showed a peak at55° C. at 10 minutes and a slight decline to 40° C. by 60 minutes,followed by a precipitous drop to 20° C. However, the PTC did not lastas long as desired with the battery, suggesting that battery selectionwould need to be considered.

Example 10 Battery Selection

A simple circuit includes the heating element and the battery. Ascurrent flows through the resistor, or heating element in the design, itheats up and delivers that heat to the bone wax in the bayonet.

The relationship between voltage (V), current (I), and resistance (R) inthe circuit is governed by Ohm's Law, which states that V=IR. Thus, fora 10 Ohm thermofoil and a 9 volt power source, the current through thecircuit is 900 milliAmps. A battery's working capacity can be describedby its number of amp-hours, which is how long the battery can output acertain current. For example, a 9 volt battery with a 450 milliAmp-hour(mAh) capacity that is connected to a 10 Ohm resistor will last 30minutes, for an ideally constant voltage and resistance.

However, as a battery is used, the amount of charge between the positiveand negative terminals decreases over time, leading to a decreasedvoltage. This decreased voltage will cause a decreased current throughthe resistor, which in the design, is critical for heat generation.Thus, the ideal battery for this design would have 1) a high enoughvoltage to provide heat, 2) a high capacity to deliver heat for an hour,3) have a low voltage drop over the course of a suitable time period,and 4) be small enough to not impede the device's functionality.

Initial tests with a 9 volt alkaline battery, chosen due to their highvoltage for a relatively small size, allowed the thermofoil andcontroller combination to reach a steady state temperature for over anhour. However, to reduce the number of components on the device, the PTCelements were used. Because these PTC elements start with a very lowresistance, a high amount of current is drawn initially to heat theelement, causing the batteries to quickly deplete (both Temp and Voltagedrop by 55 minutes). Thus, different battery configurations and newbatteries, especially lithium batteries, were investigated.

By placing multiple batteries together in certain configurations, thepower source specifications could be increased. If two batteries areconnected in series, where the positive terminal of one battery connectsto the negative terminal of another, the total voltage will be the sumof the two batteries, while the capacity remains the same. If twobatteries have both positive or both negative terminals connected, thebatteries are connected in parallel, which keeps voltage the same, butdoubles the capacity. Due to size constraints, however, theseconfigurations could not be implemented well to achieve the same voltageand capacity as 9 volt alkaline batteries did.

Lithium batteries were rated with much higher capacities than theiralkaline counterparts, so theoretically, smaller batteries could be usedto obtain the same voltage and battery lives. Preliminary testsindicated that these batteries, however, actually performed worse intemperature and battery lifetimes. The comparison is shown in FIG. 18.

The voltage of these batteries dropped to very low values once connectedto the PTC load. Further analysis of the batteries noted that standardlithium batteries are not made for sustained high-current loads, such asthe PTC elements used here.

The battery found to work well with the PTC is the Energizer LA522.Although it is a 9V lithium battery, it is designed for high currentdraw, and testing clearly demonstrated its functionality.

While the basic properties of bone wax have remained the same since itsfirst use, unfortunately, so has our application of it. Bone wax handleddirectly from packaging, at room temperature, is not amenable to beused, rather it requires the surgical technician to mend it into a morepliable form. During testing, it was discovered that heating the waxincreases its malleability such that for the first time bone wax can beapplied with a greater degree of precision in microsurgical andminimally invasive approaches. Consequently, this translates to aquicker, easier, and safer application as less force is required fromthe surgeon to intercalate it into the trabecular bone. Furthermore,since the wax is more directly applied, less wax is used in shortertime—translating to time and cost saving.

Example 11 Advantages of the Present Invention

Detailed drawings of the prototype exemplar of the invention arefeatured and described herein. The bone wax applicator of the presentinvention fulfills two key aspects of bone wax application: 1) Using aheating element softens the wax to allow for more precise and effectiveapplication and 2) The applicator is specifically designed to be used inminimally invasive/microsurgical conditions. Current use of a finger orsurgical instrument is clumsy and obstructs the surgeon's view, whereasthe minimally invasive applicator of the present invention provides asolution to a long-standing problem.

In addition, the applicator includes an ergonomically favorable handgrip, which acts to control the mechanism for wax delivery. The gripallows the surgeon the apply wax both quickly and in a controlledmanner, using a single hand. The tip is optimized for the applicationbecause it allows for both controlled wax extrusion and preciseapplication of wax to bone. The rounded tip allows the surgeon to pushthe wax into the proper place and spread it across cut bone surfaces.

While it is believed that the existing design effectively improves thecurrent state of bone wax application, future work may be done tofurther optimize the device. For example, it is contemplated within thecurrent invention that the applicator may be manufactured to bedisposable and pre-loaded with sterile wax. In addition, battery/heatingelement combinations may be further optimized in order to achieve thedesired wax temperature as quickly as possible and maintain it for aslong as possible.

Reduction in the size of the battery to further reduce the weight of thedevice and further improve the surgeon's line of vision into theoperative field is also envisioned. If the battery size is able to bereduced sufficiently, future versions of the device may feature abattery embedded within the grip or trigger assembly. To this end, thedevice may be fitted with a simple on/off switch.

In addition, the device may be made from a combination of a variety ofmaterials including metals such as aluminum, plastics, or othermaterials in order to minimize the manufactured cost of the finalcommercially sold device.

Finally, it is important to note that this technology may be readilyadapted to aid in the application of a number of other surgicalmaterials used such as bone cement, resorbable biomaterials, or otherwax-like materials all of which are embraced by the present invention.

1. An apparatus for the application of a surgical material or substanceat an elevated temperature, comprising: a) an applicator body, said bodycomprising: (i) a rear grip, (ii) a chamber for receiving said surgicalmaterial or substance, and (iii) a bayonet; b) a trigger assembly,pivotally actuable on the applicator body; c) an extrusion rod forengagement with said applicator body; d) a tip attached to the bayonet;and e) a heating assembly for warming the surgical material or substanceabove room temperature, wherein the heating assembly comprises a heatingelement and a power source operably connected to said heating element,wherein the heating element is disposed about or along at least 10percent of the length of the bayonet.
 2. The apparatus of claim 1,wherein the surgical material or substance is selected from the groupconsisting of sealants, adhesives, therapeutic compositions, creams,ointments, gels, bone, hydroxyapatite and bone wax.
 3. The apparatus ofclaim 1, wherein the bayonet is from between 10 cm and 30 cm in lengthand comprises internal diameter of between 0.1 cm and 1.0 cm andexternal diameter of between 0.5 and 1.5 cm.
 4. The apparatus of claim3, wherein the shape of the internal and external bayonet surfaces arethe same.
 5. The apparatus of claim 3, wherein the shape of the internalbayonet is round.
 6. The apparatus of claim 3, wherein the shape of theexternal bayonet is a polygonal having between 3-8 sides.
 7. Theapparatus of claim 3, wherein the tip is formed by constriction of theterminus of the bayonet.
 8. The apparatus of claim 7, whereinconstriction produces a flattened surface and slit-like tip opening. 9.The apparatus of claim 1, wherein the tip is reversibly attached to thebayonet by means of a joint, hinge, slot, groove, whorl, screw, snap, orcombinations thereof.
 10. The apparatus of claim 1, wherein the chamberfor receiving surgical material or substance comprises a cover.
 11. Theapparatus of claim 10, wherein the chamber cover is a hatch.
 12. Theapparatus of claim 11, wherein the hatch is hinged.
 13. The apparatus ofclaim 11, wherein the hatch is slides over the chamber opening.
 14. Theapparatus of claim 1, wherein the applicator body further comprises amounting platform.
 15. The apparatus of claim 1, wherein the heatingassembly comprises a heating element and a power source operablyconnected to said heating element.
 16. The apparatus of claim 15,wherein the heating element is selected from the group consisting of awire, a positive thermal coefficient (PTC) heating element andcombinations thereof.
 17. The apparatus of claim 16, wherein the heatingelement is a PTC heating element and the power source is selected fromthe group consisting of a battery and an external source operablytethered to the apparatus by a power cord.
 18. The apparatus of claim15, wherein the heating assembly further comprises one or more layers ofinsulation.
 19. The apparatus of claim 18, wherein the insulationcomprises a material selected from the group consisting of plastic,polyester film, polyethylene terephthalate, and combinations thereof.20. The apparatus of claim 18, wherein the insulation is at least twolayers of plastic disposed about the external surface of the heatingelement.
 21. The apparatus of claim 20, further comprising at least onelayer of heat shrink tubing disposed about the external surface of theinsulation.
 22. The apparatus of claim 1, wherein the trigger assemblycomprises a front grip having a shaft for receiving a spring loadedhook, said spring loaded hook having a hook member for engaging theextrusion rod and a lever for disengaging said spring loaded hook, agrip attachment means disposed through a groove of the spring loadedhook for pivotally interconnecting the front grip to the applicatorbody, and a spring for supporting the spring loaded hook.
 23. Theapparatus of claim 1, wherein the extrusion rod comprises a first regionand a second region, wherein said first region engages with theapplicator body, the second region is notched about its external surfaceand engages with the trigger assembly.
 24. The apparatus of claim 23,wherein the extrusion rod contains a third region juxtaposed to saidsecond region and being is identical to said first region.